The classic radiographic or "X-ray" image is obtained by situating an object to be imaged between an X-ray emitter and an X-ray detector. Emitted X-rays pass through the object to strike the detector, with the response of the detector varying over its area as a function of the intensity of the incident X-rays. Since the intensity of the X-rays incident on the detector is largely a function of the density of the object along the path of the X-rays, the detector receives a shadow image of the object which may then be viewed and analyzed by X-ray technicians, e.g., radiologists. In the case of analog radiographic systems, the detector is formed of X-ray film, whereas digital radiographic systems have solid-state detector components (e.g., scintillator/photodiode arrays) whereby the image is provided in electronic form.
It is common in both analog and digital radiographic systems to incorporate automatic exposure controls (AEC) which deactivate the X-ray emitter when a predetermined X-ray dose is delivered to the object, and/or when the X-ray detector has achieved the optimal optical density/signal-to-noise ratio for imaging. The AEC generally provides a dose monitor within the path of the emitted X-rays wherein an. array of dose sensors (e.g., ion chambers) is located across the dose monitor's area. Like the X-ray detector, these dose sensors receive the emitted X-rays and provide a response which varies as a function of the intensity of the incident X-rays. The response signals from the dose sensors can be compared to reference measurements in the AEC controller, and if the dose sensor signals reach or exceed reference thresholds, the AEC controller signals the X-ray emitter to cease X-ray emission. During this process, the AEC controller takes into account the settings of the emitter (its voltage/current, etc.), positioning parameters (the location of the emitter, its radiation field size/collimation, etc.), and other such variables so as to provide desirable reference thresholds for the conditions at hand.
FIG. 1 illustrates an exemplary arrangement of this type in greater detail, wherein an object 10 to be imaged is situated between an X-ray emitter 12 and an X-ray detector 14, and wherein a dose monitor 16 is situated in front of the detector 14 (though it is noted that a dose monitor may be provided within the detector itself rather than separately). The dose monitor 16 communicates with an AEC controller 18, which in turn communicates with the emitter 12 and with a workstation 20 which may include system controls, image acquisition and processing apparata, display apparata for the image, etc. FIG. 2 then illustrates the detector 14 and dose monitor 16 in greater detail, with the dose monitor 16 being depicted as an ion chamber having a number of dose sensors (ion chamber cells) 22, 24, and 26 whose signals may be selectively summed in a preamplifier 28 to provide an integrated dose monitor signal 30 for comparison with the reference measurements. Selective summing of the dose sensor signals is used because only certain sensors may be active depending on the type of imaging application at hand; for example, a standard posteroanterior (PA) chest projection imaging application might select left and right sensors 22 and 26 and deselect the central sensor 24, whereas a lateral chest exam imaging application might select only the central sensor 24 and deselect the left and right sensors 22 and 26. The deselected sensors are not used for determining the proper exposure time in the imaging application being applied because they may not provide an accurate dose measurement at the area of interest.
In digital radiographic systems, the AEC arrangement might be useful for purposes other than simply setting a desired exposure time. If a digital X-ray detector is correctly calibrated, the mean gray level of an image is proportional to the entrance exposure of the detector. Since the dose monitor also rests within the path of the X-rays, the mean image gray level should also be proportional to the dose monitor exposure. Therefore, one might be able to use the dose monitor exposure to predict the mean image gray level of the detector. If the predicted mean gray level is then compared to the actual mean gray level and is found to have a significant prediction error, this can indicate that the performance of the imaging system is diminished somewhere along the imaging chain (i.e., in the detector and/or dose monitor, etc.) and that recalibration or other maintenance is required. Thus, one might be able to utilize the dose monitor to effect a convenient monitoring system for system performance.
However, such a performance monitoring system would not work well in most radiographic imaging applications because the dose monitor and detector responses are functions of X-ray beam quality. If the X-ray beam quality changes between the dose monitor and detector owing to different imaging applications or owing to the qualities of the X-rayed object, the dose monitor will obviously not provide an accurate prediction of the mean image gray level of the detector. As an example, X-rayed objects having different sizes, shapes, and materials will produce different amounts of X-ray scatter, and the scattered X-rays will strike the dose monitor and detector to different degrees and in different locations. Further, unless the X-ray beam is of ideal quality (i.e., uniform intensity over its area), different dose monitor sensors may receive different amounts of radiation. As a result, the "averaging" provided by the selective summing process may not allow the correct prediction of mean image gray level. Therefore, it is impossible to calibrate the AEC to accurately predict mean detector image gray level for all applications, and a performance monitoring system such as the one described above will only work in idealized situations.